Fender tele dating
In this case it stands for the 3rd week of 1980 and was a Thursday.
Ignore all the "FRR" stamps you'll find on the neck base and in the neck pocket.
The third pair of digits are definately known to refer to the year.
In this example the pickups were wound by operator #20 in the 23rd week of 1978.
The first digit is supposed to reflect the year of manufacture, but there are major overlaps in this regard, and . An important thing to look for are the matching serial number stickers in the neck pocket and on the pickguard.